Module ids

Newtypes so an adapter id, a subscription id, and a message id cannot be mixed at the call site.

None of these are validated. An empty AdapterId is accepted and will show up as a blank name in logs.

Structs

• AdapterId
  Id of one running adapter instance.
• MessageId
  Id assigned to an crate::Envelope when it is constructed.
• SubId
  Id of one subscription inside one adapter.